1. Field of the Invention
The present invention relates to backlight systems for use in electronic display devices having light sources, such as liquid-crystal displays, and particularly to a backlight system with light-source arrangement, connection, power feeding, and uniformities in light emission from the display surface.
2. Description of the Background Art
FIG. 19 is a schematic diagram showing an example of a conventional backlight system for use in an electronic display device having a light source, e.g. a liquid-crystal display. In this backlight system, a plurality of parallel-arranged straight CCFL (Cold Cathode Fluorescent Tube) lamps 1, or linear lamps, are accommodated in a lamp house 3, with a single-side power-supply type inverter device 5, as a power-supply unit, placed on one side of the straight CCFL lamps 1; the single-side power-supply type inverter device 5 is connected to hot-side terminals 7 formed at one end of the straight CCFL lamps 1. Cold-side terminals 9, or the lower-voltage side, are formed at the other end of the straight CCFL lamps 1.
FIG. 20 shows the temperature distribution in the vertical direction (bottom-to-top direction) of the backlight system shown in FIG. 19, FIG. 21 shows the temperature distribution in the horizontal direction (left-to-right direction on the screen), FIG. 22 shows the brightness distribution in the vertical direction (bottom-to-top direction), and FIG. 23 shows the brightness distribution in the horizontal direction (left-to-right direction). FIG. 24 is an enlarged view showing straight CCFL lamp 1 and single-side power-supply type inverter device 5 in the backlight system of FIG. 19.
As shown in FIGS. 19 and 24, the single-side power-supply type inverter device 5 applies an ac voltage to hot-side terminal 7 of horizontally-placed straight CCFL lamp 1, causing an ac current (tube current) to flow from the hot-side terminal 7 to the cold-side terminal 9, so as to cause the straight CCFL lamp 1 to emit light. At this time, the voltage applied to the hot-side terminals 7 is a high voltage from several hundred Vrms to one thousand Vrms, while the cold-side terminals 9 are at a low voltage of nearly 0 Vrms.
In FIG. 24, reference numeral 11 denotes the tube current flowing at hot-side terminal 7 and reference numeral 13 denotes the tube current flowing at cold-side terminal 9.
The lamp house 3 is made of metal, e.g. aluminum or its alloy; since such a high voltage is applied to straight CCFL lamps 1 as mentioned above, stray capacitances 15 form in the space between the lamp house 3 and the straight CCFL lamps 1 and the lines connecting from single-side power-supply type inverter device 5 to straight CCFL lamps 1.
In FIG. 20, the temperature is relatively higher on the upper side of the display screen than on the lower side because of external factors, such as heat generation in the liquid-crystal display driving circuitry.
Techniques that use U-shaped tubes as backlights are also disclosed (for example, refer to Japanese Patent Application Laid-Open Nos. 7-288023 (1995) and 2002-278471).
In the conventional backlight system above, it is necessary to dispose the single-side power-supply type inverter device 5 in the vicinity of the hot-side terminals 7 or power-supply terminals. Therefore, as shown in FIGS. 20 and 21, the temperature is elevated to the side of hot terminals 7 (at the right in FIG. 20 and the left in FIG. 21) due to heat generation from the single-side power-supply type inverter device 5. Such temperature difference between the electrodes 7 and 9 of straight CCFL lamps 1 causes mercury to concentrate in the lower-temperature portion and then sputtering occurs at mercury-lacking terminals, which shortens the life of lamps.
Also, when stray capacitance 15 (FIG. 24) causes leakage current 17, then, as shown in FIG. 25, the tube current 13 near the cold-side terminals 9 becomes lower by the leakage current 17 than the tube current 11 near the higher-voltage hot-side terminals 7. This causes large difference in brightness particularly in the left-to-right direction on the screen (FIG. 23).
The leakage current 17 flowing through stray capacitance 15 is a reactive current that does not contribute to light emission of the straight CCFL lamps 1, and larger exciting current flows through the transformer's primary winding 19 of the single-side power-supply type inverter device 5. This leads to a reduction in power efficiency of the single-side power-supply type inverter device 5.
These problems arise also with such U-shaped tubes as are disclosed in the two Patent Documents cited above.